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Hydrogen Production by Direct Contact Pyrolysis

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Hydrogen Production by Direct Contact Pyrolysis HYDROGEN CYCLE EMPLOYING Thermochemical Considerations for the UT-3 Cycle CALCIUM-BROMINE AND ELECTROLYSIS The significant aspects of the thermodynamic operation of the Richard D. Doctor,1 Christopher L. Marshall,2 UT-3 cycle can be discussed with reasonable assurance. The and David C. Wade3 developers of the UT-3 process have observed that in this series of 4 reactions, the “…hydrolysis of CaBr2 is the slowest reaction.” 1Energy Systems Division 2Chemical Technology Division [1] Water splitting with HBr formation (730ºC; solid-gas; 3 Reactor Analysis and Engineering Division UGT = +50.34 kcal/gm-mole): Argonne National Laboratory 9700 South Cass Ave. CaBr2 + H2O JCaO + 2HBr [Eqn. 1] Argonne, IL 60439 Hence, there is the greatest uncertainty about the practicality of this Introduction cycle because of this reaction. The Secure Transportable Autonomous Reactor (STAR) project is [2] Oxygen formation occurs in an exothermic reaction (550ºC; part of the U.S. Department of Energy’s (DOE’s) Nuclear Energy solid-gas; UGT = -18.56 kcal/gm-mole): Research Initiative (NERI) to develop Generation IV nuclear reactors that will supply high-temperature heat at over 800ºC. The CaO + Br2 JCaBr2 + 0.5O2 [Eqn. 2] NERI project goal is to develop an economical, proliferation- In reviewing this system of reactions, there is a difficulty inherent resistant, sustainable, nuclear-based energy supply system based on in the first and second stages [Eqns. 1 and 2]. This difficulty is a modular-sized fast reactor that is passively safe and cooled with linked to the significant physical change in dimensions as the heavy liquid metal. STAR consists of: calcium cycles between bromide and oxide. The CaO has a cubic • A combined thermochemical water-splitting cycle to structure (a = 4.81 Å) that must undergo a dimensional change to generate hydrogen, accommodate the CaBr2 orthorhombic structure (a = 6.58 Å; b = • A steam turbine cycle to generate electricity, and 6.87 Å; c = 4.34 Å). This process must then be reversed. As the • An optional capability to produce potable water from calcium reactant undergoes this change in dimensions, sintering brackish or salt water. will occur unless the calcium is carefully dispersed on a suitable support. Recent efforts by Sakurai et al. have considered pellets However, there has been limited reporting on critical elements of with the CaO supported on CaTiO3 at CaO:CaTiO3 ratios between the thermochemical cycle: (1) establishing chemical reaction 0.5 and 2.5 Sakurai reported plugging of pore volumes as the cycle kinetics and operating pressures and (2) addressing materials issues is reversed and the CaBr2 is regenerated. We intend to investigate for hydrogen production. This paper reviews the thermodynamic suitable support structures for the calcium that will tolerate this basis for a three-stage Calcium-Bromine water-splitting cycle based cycling. on the University of Tokyo Cycle #3 [UT-3] and discusses the further chemistry work that is required to develop an economical After recovering the HBr, the UT-3 process proposes bromine process including modifying UT-3 to incorporate HBr dissociation. regeneration at 220ºC in a solid-gas reaction followed by heating to regenerate the hydrogen at 650ºC at a temperature close to the Calcium-Bromine Cycle for Thermochemical Water Splitting FeBr2 melting point of 684ºC. Beginning in the 1960s, interest in thermochemical water-splitting A steam turbine will be used to harvest reject heat (at ~550°C) and cycles for the large-scale production of hydrogen began to grow. will supply the electrical demands for pumping reagents in the Although many cycles have been published, few have been the water cracking plant, for circulating brine and water in the subject of rigorous studies based on detailed thermodynamic desalinization plant, and for pressurizing the H2 and O2 products for calculations; fewer yet have undergone laboratory testing to distribution. A desalinization system will be the final heat rejection establish kinetics and yields or to develop the chemical and path from the cascaded thermodynamic cycles. physical properties needed to complete detailed mass and energy balances. The identification of conceptual cycles must then [3] Exothermic bromine regeneration (220ºC; solid-gas; consider secondary environmental releases and special challenges UGT = -29.470 kcal/gm-mole; UHT = -65.012 kcal/gm-mole): to implementing some cycles (for example, those that employ large volumes of mercury). Fe3O4 + 8HBr J3FeBr2 + 4H2O + Br2 [Eqn. 3] Among the cycles that have the highest commercial potential, a [4] Hydrogen formation from FeBr (650ºC; solid-gas; recent screening study identified the two leading candidate cycles 2 1 as Sulfur-Iodine and Calcium-Bromine. The Calcium-Bromine UGT = 32.178 kcal/gm-mole; UHT = 91.913 kcal/gm-mole): cycle is being investigated by JAERI and has been considered in detailed technical reviews.2 At Argonne National Laboratory, we 3FeBr2 + 4H2O JFe3O4 +6HBr + H2 [Eqn. 4] are investigating a variant of this cycle that offers some advantages over the original cycle.3 It is called the “Calcium-Bromine cycle, The thermodynamics for this reaction system are favorable, and a or Ca-Br cycle,” to avoid confusing it with the excellent efforts on diagram of the Gibbs free energies for a simplified reaction the UT-3 cycle. network appears in Figure 1. In this diagram, the last two stages are reduced to the basic process of HBr dissociation. theory, is more efficient than current electrolysis pathways to hydrogen. However, considerable effort is needed to see whether 1- CaBr2 to CaO 60.00 this promise can be translated into practical operating systems. A ) 3-stage system employing Calcium-Bromine combined with e 50.00 2- CaO to CaBr2 electrolysis offers a potentially attractive route to the early 40.00 deployment of this technology. As successful hydrogen producing m-mol 3- HBr dissoc. g 30.00 / thermochemical cycles emerge, nuclear power can play a 20.00 significant role in mitigating climate change and seems the only H2O electrolysis 10.00 viable carbon-free route to supplying massive quantities of hydrogen that are needed for the transportation sector. ergy (kcal 0.00 n -10.00 0 500 1000 1500 Acknowledgements -20.00 Free E -30.00 This effort was sponsored by the U.S. Department of Energy’s Temperature (K) (DOE’s) Nuclear Energy Research Initiative at Argonne National Laboratory under Contract No. W-31-109-Eng-38 with DOE. The Figure 1. UG vs. Temperature (oK) for the Calcium-Bromine assistance of M. Serban in preparing this preprint is appreciated. 3-stage cycle contrasted with water electrolysis shows that the 3rd stage of HBr dissociation has a 48% lower electricity requirement. References The Ca-Br Cycle – a Modified UT-3 Cycle 1. Besenbruch, G.E., et al., “High Efficiency Generation of Hydrogen Fuels Using Nuclear Power,” The First Information We propose to employ a modified UT-3 cycle – with a single-stage, Exchange Meeting on Nuclear Production of Hydrogen, Paris, rather than a two-stage, HBr-dissociation step. Here, the hydrogen France – October 2000, OECD (2001). formation will employ either commercial HBr electrolysis or the 2. Tadokoro, Y., and O. Sato, “Technical Evaluation of Adiabatic use of a plasma chemistry technique operating near ambient UT-3 Thermochemical Hydrogen Production Process for an conditions. Process conditions for the plasma-chemical approach Industrial-scale Plant,” JAERI Review 98-22; 10.3 (1998). are ~100°C; gas phase; UGT = +27.32 kcal/gm-mole: 3. Wade, D.C., et al., “A Proposed Modular-Sized, Integrated Nuclear and Hydrogen-based Energy Supply/Carrier System,” 2HBr + plasma Q H2 + Br2 [Eqn. 5] The First Information Exchange Meeting on Nuclear Production The reasons for adopting this strategy can be seen from of Hydrogen, Paris, France, October 2000, OECD (2001). consideration of the Gibbs free energies for this cycle. This stage 4. Sakurai, M., et al., “Analysis of a Reaction Mechanism in the takes advantage of power requirements that now are lowered to UT-3 Thermochemical Hydrogen Production Cycle,” 48% of those necessary for water electrolysis (where UGT [H2O] = International J. of Hydrogen Energy, Vol. 21, No. 10, +56.70 kcal/gm-mole). Power draws of ~1eV are more realistic for pp. 871-875 (1996). a commercial facility: 5. Sakurai, M., et al., “Improvement of Ca-pellet Reactivity in the UT-3 Thermochemical Hydrogen Production Cycle,” e- + HBr +3.5eVbond Q H0 + Br –3.4eV [Eqn. 6] International J. of Hydrogen Energy, Vol. 20, No. 4, pp. 297-301 (April 1995). Losing the electron to the bromine is economically unacceptable; 6. Gutsol, A.F., and J.A. Bakken, “A New Vortex Method of hence, efforts that began at the Kurchatov Institute (Moscow, Plasma Insulation and Explanation of the Ranque Effect,” J. Russia) during the 1980s work with plasma-chemical systems so Physics D: Applied Physics, Vol. 31, No. 6, 1998, pp. 704–711. that the products of the dissociation do not recombine. Advances 7. Gutsol, A.F., and V.T. Kalinnikov, “Reverse-Flow Swirl Heat in this technique employing “reverse-vortex flow” have recently Insulation of Plasma and Gas Flame,” Teplofizika vysokikh been reported.6,7 Applying this technique will result in a small temperatur, Vol. 37, No. 2, 1999, pp. 194–201 (High draw on the electric power from the system to produce a cold Temperature Physics, Vol. 37, No. 2, 1999, pp. 172–179). plasma-chemical reaction. 8. Funk, J.E., and R.M. Reinstrom, “Energy Requirements in the Production of Hydrogen from Water,” Industrial and With minor energy recovery in the oxygen-formation stage Engineering Chemistry Process Design and Development, [Eqn. 2], the calcium-bromine cycle has a 66% ideal efficiency, Vol. 5, No. 3 (July 1966), pp. 336–342. defined as:8 9. Besenbruch, op. cit. Efficiency – ideal = UHCycle/UGCycle*(Treactor – Tambient)/Treactor The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory (“Argonne”) Practical considerations for the UT-3 cycle suggest an efficiency in under Contract No.
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